Inflexible learning, the inability to change from one course of action to another by learning from a behavioral consequence, is a common symptom of many psychiatric disorders. Its biological mechanisms are largely unknown, but one important epigenetic cause is brain-derived neurotrophic factor (BDNF), a major neuronal growth factor in the brain. BDNF deficiency in the hippocampus (HIP) and medial prefrontal cortex (mPFC) causes inflexible learning. Stress reduces BDNF levels in these regions via epigenetic inactivation of promoter IV, a major activity-dependent BDNF promoter. Reduced BDNF levels and inactive promoter IV are observed in psychiatric patients. Extensive studies have elucidated the mechanisms underlying BDNF deficiency within specific brain regions. However, the neural mechanisms underlying BDNF deficiency between different brain regions remain unknown. In particular, we still do not know how BDNF deficiency affects signal processing between the HIP and mPFC during flexible learning. This knowledge gap may be attributable to the technical difficulties of manipulating BDNF in multiple brain regions and in measuring neural functions across brain regions. We have addressed these issues by generating mutant mice (KIV) that lack promoter IV-driven BDNF and show inflexible learning. We also have developed an in vivo electrophysiological system that allows simultaneous recording and stimulation of multiple brain regions in mice behaving in a smell-taste flexible learning test. Our long-term goal is to elucidat the neural mechanisms of inflexible learning caused by BDNF promoter IV deficiency. Meeting this goal will help to explain the pathophysiology underlying many psychiatric disorders arising from stress that inactivates promoter IV. We recently found that BDNF promoter IV deficiency reduces long-term potentiation (LTP), a cellular form of memory, in the HIP, but enhances LTP in the mPFC. How do these opposing effects of BDNF deficiency affect the signal processing between the HIP and mPFC, and how does this relate to flexible learning? From our preliminary results, we hypothesize that BDNF deficiency impairs the normal suppression of mPFC responses to input from the HIP during breaks between flexible learning tasks, which reflects inflexible behavior. To explain how this occurs, we also propose a novel model: neuronal synchrony acts as a gate to control the timing of the HIP-mPFC signals in a frequency-dependent manner; BDNF deficiency reduces neuronal synchrony and thus impairs timing controls of HIP-mPFC signals. We will test these hypotheses by determining the effects of BDNF deficiency on Aim 1) HIP-mPFC signals/LTP and Aim 2) neuronal synchrony during flexible learning, and by determining timing relations among HIP-mPFC signals, neuronal synchrony, and correct behavioral responses. Successful completion of this project will elucidate the timing-dependent neural mechanisms of inflexible learning, and will expand the understanding of learning mechanisms from synaptic plasticity within single brain regions to timing-dependent signaling across brain regions.